Lithium Abundances of Nearby Young Solar-Type Stars Based on Optical High-Resolution Spectroscopy

A&A 537, A91 (2012) Astronomy DOI: 10.1051/0004-6361/201117133 & c ESO 2012 Astrophysics

Lithium abundances of nearby young solar-type stars based on optical high-resolution spectroscopy

L.-F. Xing1 and Q.-F. Xing2

1 Department of Technology and Physics, Zhengzhou University of Light Industry, 5 Dongfeng Road, 45002 Zhengzhou, PR China e-mail: [email protected] 2 National Astronomical Observatories, Chinese Academy of Sciences, 100012 Beijing, PR China Received 25 April 2011 / Accepted 4 November 2011

ABSTRACT

Context. Lithium has long been recognized as a powerful tool for investigating the internal mixing of low-mass stars. In addition, accurate measurements of lithium abundances in young solar-type stars provide independent and reliable age diagnostics. Aims. To study the relationship between lithium and activity in normal solar type-type stars and determine the effectiveness of lithium and kinematics as activity indicators, we perform a lithium survey of nearby late-type stars. We search for nearby young solar-type stars and investigate the relationship between lithium abundances and X-ray activity by measuring log Lx. Methods. On the basis of high-resolution spectroscopic observations, we derived the lithium abundances of 33 young solar-type stars by comparing the measured Li I λ 670.8 nm equivalent widths with the curve of growth calculations in non-LTE (NLTE) conditions. Results. We obtain the lithium abundances of a sample of young solar-type stars. Using the lithium abundance and X-ray luminosity, Lx, we investigate the relationship between Li abundance and X-ray activity. We find a clear correlation between lithium abundance (log N(Li)) and X-ray luminosity, log Lx, for our sample stars. Conclusions. As the X-ray luminosity, Lx, becomes stronger, the lithium abundance decreases in our sample (11 pre-main-sequence, 15 zero-age-main-sequence, and 7 young main-sequence) stars. The results imply that there is a large number of solar-type stars. A correlation appears to exist between Li abundance and age, thus confirming the presence of very active young stars close to the Sun. Key words. stars: evolution – stars: abundances – stars: solar-type

1. Introduction the amount of Li depletion in a star (Barrado y Navascués & Stauffer 1996;Jonesetal.1997). In addition, there is clear Solar-type stars are low-mass, late spectral-type stars. They are evidence of a correlation between rotation rates and Li abun- similar to the Sun in mass and evolutionary state. Physically, dances in the Pleiades and Hyades, with the fastest rotators hav- they have a broadly similar structure and a convective envelope; ing the highest Li abundance (Tschäpe & Rüdiger 2001; Rebolo however, they do not have completely dominant convection, as & Beckman 1988). In particular, the X-ray surface flux is a found in M dwarfs. Young solar-type stars (a weak-line T Tauri good measure of stellar activity because it shows a clear de- star (WTTS), and low-mass zero-age-main-sequence (ZAMS), pendence on stellar rotational velocity (Preibisch 1997). The en- and young main-sequence (MS) stars) display various forms of hanced X-ray emission displayed by rapid rotators (T Tauri stars activity caused by dynamic processes. and RS CVn systems) relative to main-sequence stars can be Lithium has long been recognized as a powerful tool for in- accounted for by their higher rotational velocities (Bouvier & vestigating the internal mixing of low-mass stars. Since lithium Bertout 1985). Therefore, a correlation can be expected between isotopes in low-mass stars are destroyed by proton capture at low lithium abundances and star activity. temperatures, they allow us to directly probe the depths of outer- To study the relationship between lithium and X-ray activity mixed envelopes. Observations of field stars and open clusters in normal solar-type stars, as well as determine the effectiveness clearly show that the destruction rate of Li depends on the mass, of lithium and kinematics as activity indicators, we performed a age, and chemical composition of a star (Duncan 1981; Spite & survey of lithium over a sample of nearby dwarfs. The aim of the Spite 1982; Duncan & Jones 1983;Cayreletal.1984; Boesgaard current paper is to investigate the relation of lithium abundances & Tripicco 1986; Hobbs & Pilachowski 1986; Rebolo et al. to X-ray activity in terms of log LX. The observations and data 1988). In particular, different authors have proposed empirical reduction are presented in Sect. 2. The analysis of lithium abun- expressions for Li abundance as a function of star age (Rebolo dance is provided in Sect. 3, and the results and discussion are 1989; Boesgaard 1991). presented in Sect. 4. Increasing evidence has indicated that dispersion in Li abundances exists among stars with similar ages, metallicities, and 2. Observations and data reduction masses (Soderblom et al. 1993a;Boesgaardetal.1998). This dispersion suggests that some other stellar property is important Spectroscopic observations were carried out for thirteen nights in determining the amount of Li depletion in stars. Observational from January 15 to 17, 2008, October 6 to 8, 2009, January 26 to data clearly indicate that rotation plays a key role in determining 28, 2010, and February 18 to 21, 2011 with the Coude Echelle Article published by EDP Sciences A91, page 1 of 6 A&A 537, A91 (2012)

HD 141004

Li I

CaI HD 126868

Li I

CaI HD 81025

Li I

CaI HD 82210

Li I CaI

Fig. 1. Examples of the spectra of stars with a range of Li I λ 6707 mÅ Fig. 2. Comparison of our measurement of the Li equivalent width ob- lines. tained with a Gaussian fit and similar measurements obtained by other authors for the same stars (Christian & Mathioudakis 2002;Wichmann et al. 2000; Gregorio-Hetem & Hetem 2002; de Laverny et al. 2003; Spectrograph and a 1024×1024 Tek CCD attached to the 2.16 m Takeda & Kawanomoto 2005;Jasniewiczetal.1999; Montes et al. telescope at the Xinglong station of the National Astronomical 2001). Observatories, Chinese Academy of Science (NAOC). The red arm of the spectrograph with a 31.6 groove/mm grating was used in combination with a prism as the cross-disperser, which pro- three strong lines included in our spectral range (Ni I 664.36 nm, vided good separation between the echelle orders (Zhao & Li Fe I 666.34 nm, and V I 662.48 nm) for all target stars. We then 2001). With a 0.5 mm slit (1.1 arcsec), the resolving power was compared them with those measured in the spectra of stars of of approximately 37 000. The exposure time was chosen to en- similar temperature in IC 2391 and IC 2602, which were old sure that the signal-to-noise ratio (S/N) was higher than 100, and enough (30–50 Myr, Randich et al. 2001) to ensure that their ff the spectral coverage is 580–880 nm. This relatively high reso- spectra are not a ected by veiling. Therefore, we did not attempt lution was judged to be important in view of the relatively weak any veiling correction in our young solar-type stars. lithium lines in some stars. In Fig. 2, we compare our measurement of the Li EWs with First, all observational data were reduced using the Image those obtained by other authors for the same stars (Christian & Reduction and Analysis Facility (IRAF)1software package in Mathioudakis 2002;Wichmannetal.2000; Gregorio-Hetem & the standard fashion, including image trimming, bias subtrac- Hetem 2002; de Laverny et al. 2003; Takeda & Kawanomoto tion, flat-field division, spectrum extraction, and cosmic ray re- 2005;Jasniewiczetal.1999; Montes et al. 2001). The agreement moval. The wavelength calibration was then obtained by taking between the values was good; however, for small EWs, we noted ff the spectra of a Th-Ar lamp. Finally, all spectra were normal- some di erences between the samples of other authors and our ized using a cubic spline fit to the observed continuum. Figure 1 results. We found an agreement of closer than 15% for 8 stars shows the spectra of four sample stars in the range of the Li I λ when compared with the literature data, and we estimated an 670.8 nm line. average error bar of 5% in the EW values at 1σ, resulting mainly To determine the Li I λ 670.8 nm equivalent widths from uncertainties in the continuum placement. (EW(Li)), all spectral lines of the relevant order were averaged ff to find regions in the continuum una ected by metal lines. These 3. Lithium abundances of young solar-type stars regions were then used to fit the continuum with a straight line. The EW (Li) was then determined by fitting a Gaussian curve. 3.1. Effective temperatures and surface gravity The nominal resolving power λ/λ was 105,whichwassufficient The effective temperature of our 33 sample stars was determined to resolve the Li I feature from a nearby Fe I line at 6707.44 Å in from the line-depth ratios of VI λ 6251.83 and Fe I λ 6252.527, all regions. Since we observed bright stars and the seeing con- using the calibrations of Gray and Johanson (1991). The line S/N ditions were generally good, the of our observations was depths were measured with a ruler on a plot of the profiles (Xing high, between 100 and 300. 2010). We also determined the effective temperature for 33 pro- Following Palla et al. (2005), the relationship between − = gram stars from the (B V) color index, using the calibrations of the true and measured equivalent widths (EWs) is EWtrue Casagrande et al. (2006). The effective temperature of program − r r EWmeas(1 ), where is the ratio of the excess to the pho- stars derived from different methods is listed in Table 1. r tospheric continuum. To estimate , we measured the EWs of The gravity is determined via Hipparcos parallaxes accord- 1 IRAF is distributed by the National Optical Astronomy ing to the method given by Nissen et al. (1997). Observatories,which is operated by the Association of Universities for Following the uncertainty analysis of Chen et al. (2000), we Research in Astronomy, Inc., under contract with the National Science can estimate the mean uncertainty in log g. The largest uncer- Foundation. tainty in the gravity comes from the parallax. A typical relative

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Table 1. Photometric results and other parameters of our sample of stars.

− C R #StarsSpT.VBVTeﬀ Teﬀ EW(Li) log N(Li) EW(Hα)logLx log Fs (K) (K) (mÅ) (mÅ) (erg/s) 1BD+61 119 F8 8.98 0.590 5948 5939 132 3.01 0.77 28.15 6.90 2 RX J0156.4+1224 G 10.35 0.598 5956 5961 284 3.14 0.38 28.46 6.89 3BD+30 397A K8 10.12 1.205 4398 4380 215 2.23 0.03 29.95 6.24 4 HD 17662 G5 8.46 0.720 5542 5622 23 1.89 –0.56 32.00 6.76 5BD+ 21 418 G5 8.5 0.635 5843 5895 247 3.18 0.99 29.21 6.85 6 HD 20678 K0V 7.954 0.728 5268 5263 33 1.49 1.09 29.04 6.75 7 HD 21051 KIII 6.05 1.187 4817 4826 287 2.76 1.18 30.07 6.25 8 HD 21845 K2 8.250 0.800 5068 5076 210 2.88 0.62 30.08 6.67 9 HD 26913 G8V 6.92 0.680 5662 5648 85 2.51 1.35 30.67 6.80 10 HD 284503 G8 10.48 0.872 5109 4997 161 2.61 0.01 29.79 6.60 11 HD 285840 K1 4.76 0.702 5604 5598 178 3.02 0.06 30.23 6.78 12 HD 283716 K0IV 10.34 0.769 5403 5422 39 1.89 –0.28 31.48 6.71 13 HD 31281 G0 9.19 0.672 5647 5639 0 1.01 1.03 31.73 6.81 14 HD 32704 G8V 8.66 0.759 5338 5347 24 1.52 0.24 30.99 6.72 15 HD 287927 G5 10.6 0.709 5498 5507 254 3.24 0.08 29.16 6.77 16 HD 245924 K0IV 9.7 0.880 5109 5108 247 2.84 0.05 30.69 6.59 17 HD 251108 K2 9.97 1.332 4342 4339 115 1.68 –1.54 29.93 6.10 18 BD+33 1646 K7 9.90 1.400 4236 4242 15.3 0.41 –0.97 29.43 6.02 19 HD 79551 G5 8.734 0.662 5642 5635 19 1.52 0.27 31.28 6.82 20 HD 81025 G2III 6.368 0.774 5298 5306 22 1.98 1.22 30.16 6.70 21 HD 82210 G5III 4.565 0.781 5249 5244 30 1.59 1.15 30.33 6.69 22 HD 95559 K0V 8.83 0.872 5038 5043 68 1.96 0.24 30.42 6.60 23 BD+12 2343 G5 9.33 0.750 5347 5355 17 1.52 –0.04 30.87 6.73 24 HD 101501 G8V 5.32 0.723 5453 5447 25 0.86 1.07 28.22 6.76 25 HD 111395 G5V 6.31 0.703 5508 5513 29 1.36 1.13 28.41 6.78 26 BD+23 2581 K4V 9.61 1.030 4709 4716 17 0.89 1.13 30.22 6.42 27 HD 126868 G2IV 4.844 0.693 5506 5497 98 2.48 1.23 30.32 6.79 28 HD 133640 G2IV 4.76 0.647 5682 5677 24 2.01 0.99 29.69 6.84 29 HD 141004 G0V 4.43 0.604 5868 5862 14 1.96 1.37 27.67 6.89 30 HD 340540 K0 9.15 0.937 4897 4903 91 2.17 0.15 31.05 6.53 31 BD+48 3686 K1V 8.9 0.847 5092 5084 248 2.48 0.25 29.74 6.62 32 BD+66 1664 G8 8.74 0.708 5489 5482 25 1.51 0.07 32.94 6.77 33 HD 232862 G8II 9.46 0.878 5048 5052 191 2.51 0.13 30.46 6.59

Notes. C:theeffective temperature was determined from B − V colour. R:theeffective temperature was determined from the line-depth ratios of VI λ 6251.83 and Fe I λ 6252.527. error of 5% corresponds to an error of 0.04 dex in log g.The Basrietal.(1991) showed that for 5200 K and an equiva- estimated errors of 70 K in effective temperature and 0.05 mag lent width of EW(Li) = 0.40 Å, changing the microturbulence in apparent magnitude each lead to an uncertainty of 0.02 dex from 1.8 to 3.5 km s−1 only affects the derived abundances by in log g,whileanerrorof0.05M in mass lead to an error of –0.1 dex (King 1993). The uncertainty in log N(Li) is relate to 0.03 dex in gravity. Adding these values in quadrature gives a the error in log g, which is negligible based on consistent calcu- final uncertainty <0.2 dex. lations. However, to be conservative, we adopted an uncertainty of σ (log g) = ±0.1 dex in the relative Li abundances. The uncertainty in the EWs is ±10%, introducing an uncertainty ≤±0.1 dex 3.2. Calculation of lithium abundance to the abundances. An uncertainty of ±70 K in Teff results in an ≤± Lithium abundances (on a scale where log N(H) = 12.00) were uncertainty of 0.15 dex in the abundances. Adding these values in quadrature gives a final uncertainty for the relative abun- obtained from comparison of the measured Li I λ 670.8 nm ≤± EWs with the curve of growth calculations in non-LTE (NLTE) dances of 0.2 dex for all but one star (HD 31281). conditions (Pavlenko and Magazzú 1996, used the newest models (Kurucz 1993) with solar metallicity and gravities 3.3. Calculation of X-ray luminosity and effective temperatures in the range log g = 3.0–4.5 and L Teff = 3500−6000 K). X-ray activity measures, X, were calculated for each star using The lithium abundances of 33 observed sample stars are the ROSAT All-Sky Bright Source Catalogue data. The X-ray reported in Table 1. The main source of error in the de- flux, fX,wasderivedas rived log N(Li) values is the uncertainty in the effective tem- −2 −1 fX = ECF · count rate (erg cm s ) perature. Following the uncertainty analysis of Martín et al. (1994), we can estimate the mean uncertainty of log N(Li). where ECF is the energy conversion factor given as − − ± ff = + × 12 2 1 From Fig. 5, we can see that the uncertainty of 250 K in Te ECF (5.30 HR1 8.31) 10 (erg cm cts ) or 0.5 dex in log g, translates into error bars in log N(Li) of about 0.4 dex and 0.1 dex, respectively. The effects of micro- according to Schmitt et al. (1995)where 2 turbulence in the derived Li abundance are probably negligible. LX = 4πd fX

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Fig. 3. A comparison of log N(Li) of sample stars of Christian & Fig. 4. Li abundance versus effective temperature for stars in the present Mathioudakis (2002), Wichmann et al. (2000), de Laverny et al. (2003), survey. The solid and dashed curves are the fiducial log N(Li) vs. Takeda & Kawanomoto (2005), Jasniewicz et al. (1999), Montes et al. Teff curves for the Pleiades (top envelope) and Hyades, respectively (2001) with that of this work. (adapted from Deliyannis 2000). and d is the distance to the star derived from the trigonometric parallax. The trigonometric parallax of star was taken from VizieR.

4. Discussion and results For the sake of consistency, we show in Fig. 3 a comparison of log N(Li) derived in this study with other works (Christian & Mathioudakis 2002;Wichmannetal.2000; de Laverny et al. 2003; Takeda & Kawanomoto 2005;Jasniewiczetal.1999; Montes et al. 2001). As expected, we found only minor differ- BD+33 1646 ences with respect to other original lithium abundances. We are confident that our lithium abundances are reliable. Figure 4 shows the log N(Li) versus (vs.) the Teff distribution HD 111395 for young solar-type stars in our sample. The solid and dashed HD 101501 curves are the fiducial log N(Li) vs. Teff curves for the Pleiades HD 141004 (top envelope) and Hyades clusters, respectively (adapted from Deliyannis 2000). The figure indicates that most of our sample stars are above the Hyades track. Many stars in our sample are clearly quite young. We find that among the stars in our sample, 11 have a log N(Li) higher than that of the Pleiades (all of Fig. 5. Lithium abundances as a function of the X-ray luminosity which are younger than ZAMS Pleiades stars, and should be pre- (log Lx) for the sample of young solar-type stars. main sequence (PMS) stars), 15 have a log N(Li) between that of the Pleiades and Hyades (all are younger than Hyades stars, and should be ZAMS), and 7 have a log N(Li) lower than that of the Hyades (all of which should be young solar-type stars). other hand, observations and data analysis show that the rapid ro- The figure also indicates that for these young solar-type stars, no tation of young solar-type stars exhibits higher X-ray luminosity dependency of log N(Li) on Teff is discernible. (Bouvier et al. 1985;Xingetal.2007a). The mean levels of both Soderblom et al. (1993a) studied lithium abundance in con- lithium abundance and X-ray luminosity are known to decrease junction with rotational velocity and chromospheric activity in with decreasing projected rotational velocity (although perhaps the Pleiades, and found that (for 0.8 < B − V < 1.2) stars with a large dispersion) in solar-type MS stars. Therefore, some with a large EW of the Li I 6708 Å line tend to have a large level of correlation between lithium abundances and X-ray lumi- value of projected rotational velocity. In the α Per cluster, more nosity was expected in the nearby field young solar-type stars. rapid rotators also appear to have a stronger lithium abundance Figure 5 presents the relationship between X-ray lumi- (Balachandran et al. 1988;Stauffer et al. 1993). These results nosity (log Lx) and lithium abundances for the sample of 33 are the same for nearby field solar-type stars (de Medeiros et al. (11 PMS, 15 ZAMS and 7 young MS) stars selected from the 2000; do Nascimento et al. 2000; Cutispoto et al. 2003). On the ROSAT survey. Except for four multi-star system (BD+33 1646,

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abundances. The results of the present study are in good agreement with the results of Pasquini et al. (1994), Favata et al. (1993,1995), Tagliaferri et al. (1994, 2000), and Jeffries (1995). Lithium and X-ray activity appear to be correlated for the 33 young solar-type stars in our sample. These results are in good agreement with the rotation evolution model (e.g., Bouvier 1994; Soderblom et al. 1993b; Collier Cameron et al. 1995; Keppens et al. 1995), and Li could serve as a “clock” of stars that develops in the PMS phase (Drake et al. 2003). This correlation for our present sample appears to be simply an age effect, with lithium decaying and activity increasing with age for PMS solar-type stars. Our results, which show agreement with recent findings from X-ray surveys, confirm the presence of very active young stars close to the Sun.

5. Conclusions In summary, we have presented high-resolution spectroscopic observations for a sample of 33 young solar-type stars discov- ered in the X-ray wavelength range during an ROSAT all-sky survey. On the basis of our observations, we have derived the

Fig. 6. Lithium abundances as a function of the log Fs for the sample of lithium abundances of the stars by comparing the measured Li I young solar-type stars. λ 670.8 nm EWs with the curve of growth calculations in non- LTE (NLTE) conditions (Pavlenko & Magazzú 1996). We have found a clear correlation between X-ray activity and lithium HD 111395, HD 101501, and HD 141004), Fig. 5 shows abundances: as the X-ray luminosity, Lx, becomes stronger, the that there exits a clear correlation between X-ray activity and lithium abundance decreases in our sample stars. The results of Li abundance: as the X-ray luminosity, LX becomes stronger, the the current study provide additional evidence that this correla- lithium abundance decreases. tion exists for a large number of solar-type stars. A correlation Pasquini et al. (1994) found a clear tendency for nearby appears to exist between lithium abundances and the age of stars. chromospherically active solar-type stars to have high log N(Li). In addition, we have confirmed that there are very active young Similar results were found in X-ray samples or EUV-selected stars close to the Sun. active stars (Favata et al. 1993; Tagliaferri et al. 1994, 2000; Jeffries 1995). Stellar activity can then be considered a good in- Acknowledgements. We would like to thank Prof. Gang Zhao for helpfull dis- cussions. The authors gratefully acknowledge the many and detailed positive dicator of age for solar-type stars (e.g., Soderblom et al.1991; contributions made by the referee. This work was supported by the joint- Pizzolato et al. 2000). We therefore searched for a correlation ing fund of the Astronomy of the National Natural Science Foundation of between Li abundance and chromospheric activity in our sample China and the Chinese Academy of Sciences, under Grants No. 10878015 and stars. 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